A Differential Mobility Spectrometer (DMS), also referred to as a Field Asymmetric Waveform Ion Mobility Spectrometer (FAIMS) or Field Ion Spectrometer (FIS), typically performs gas-phase ion sample separation and analysis. In some circumstances, a DMS is interfaced with a mass spectrometer (MS) to take advantage of the atmospheric pressure, gas-phase, and continuous ion separation capabilities of the DMS and the detection accuracy of the MS. By interfacing a DMS with an MS, numerous areas of sample analysis, including proteomics, peptide/protein conformation, pharmacokinetics, and metabolism analysis have been enhanced. In addition to pharmaceutical and biotech applications, DMS-based analyzers have been used for trace level explosives detection and petroleum monitoring.
Differential mobility spectrometry is a variant of ion mobility spectrometry (IMS). A DMS, like an ion mobility spectrometer, is considered an ion mobility based analyzer because the DMS separates and analyzes ions based on the mobility characteristics of the ions. In an IMS, ions are pulsed into and pass through a drift tube while being subjected to a constant electric field. The ions interact with a drift gas in the drift tube and the interactions affect the time it takes for the sample ions to pass through the drift tube. The drift time and thus mobility is a function of the size, shape, and charge state of an ion and its interactions with the background gas. These interactions are specific for each analyte ion of a sample, leading to an ion separation based on more than just mass/charge ratio. In contrast, in a time-of-flight mass spectrometer (TOF-MS), there is a vacuum in the drift region of the MS and, therefore, an ion's time through the MS drift region is based on the ion's mass-to-charge ratio (m/z) in the collision-free environment of the vacuum.
A DMS is similar to an IMS in that the ions are separated in a drift gas, which is typically supplied from a gas source upstream of the DMS. However, unlike an IMS, the DMS uses an asymmetric electric field waveform that is applied between two parallel electrodes through which the ions pass, typically, in a continuous manner. RF voltages, often referred to as separation voltages (SV), are applied across the ion transport chamber, perpendicular to the direction of the transport gas flow. As a result of the SV, the ions' flight paths can deviate from the center of the chamber and migrate toward the walls unless their trajectory is corrected by a counterbalancing voltage, a DC potential often referred to as a compensation voltage (CoV) that is applied to electrodes to restore a stable trajectory for a particular ion caused by the difference between high field and low field ion mobilities. In some DMS systems, a modifier liquid can also be added to the drift gas to provide increased selectivity by clustering with ions to different degrees, thereby shifting these ions differential mobilities. In known devices, with the CoV of the DMS being set to a fixed value, ions can be continuously introduced into the DMS with only ion species with a particular differential mobility being transmitted by the DMS.